Some walls appear too bright while others appear fine
Examine your Lights for position and direction and specular value.  If you use a Sun Lamp and point it you can selectively illuminate a wall.  If you are using specular light direction and viewing angle must be considered.
Colors.  Make sure the walls share a material.  The materials would have a number 2 or greater if it is shared.
Examine thes normals of the wall.  Edit Mode. Key N. Mesh Display. Normals Click plane Face Icon.
More Drastic.  Create archive of file. Delete all lights.  Add back one by one to see if the asymmetry persists. Two antiparallel sunlights should achive symmetrical lighting.